As the method of sealing the containers with the lids, there can be, usually, exemplified a method of adhesion by applying an adhesive to the contacting surfaces of the containers and the lids, and an art of forming the contacting surfaces of the containers and the lids with a heat-sealable resin, and attaining the sealing by melt-adhering the heat-sealable resin. The melt adhesion by heat-sealing is a simple method and has, usually, been widely used.
However, the melt adhesion by heat-sealing requires extended periods of time for the step of melt adhesion and for the subsequent step of cooling, and is not satisfactory from the standpoint of productivity. Further, The heat-sealing system, usually, uses a generally employed heat-seal bar, and makes it necessary to conduct the heat from the outer surface of the melt-adhering portion onto the sealing surface requiring, therefore, an extended period of time for conducting the heat when thick containers are handled. To prevent a decrease in the productivity, therefore, limitation is imposed on the thickness arousing, therefore, a problem in that only a low degree of freedom is allowed for designing the container and lid shapes. Further, a predetermined period of time is required until the heat-sealed portion is cooled and is completely sealed. Therefore, when the content having a spontaneous pressure is contained or when the content is hot-filled, the gas in the head space that is expanded due to the heat of sealing might escapes through the sealing portion in a molten state, and the sealing might peel off.
On the other hand, as the method of melt-adhering the members of packing bodies such as containers and lids, there have heretofore been known a melt adhesion by a laser. A patent document 1, for example, proposes an art of melt-adhering a bottom lid and an upper lid to the container body as a unitary structure by melt adhesion by laser. Upon melt-adhering the parts of the packing bodies by melt adhesion by laser, the sealing surfaces can be melt-adhered without much limited by the thickness of the members and, therefore, the time required for the melt adhesion is shortened as compared to the case of the heat-sealing.
The present inventors have also proposed a method of melt adhesion by constituting a self-pressing mechanism capable of pressing the members of the packing bodies such as containers and lids onto each other, enabling the melt adhesion by laser to be attained without the need of fixing the melt-adhering portions by using the external pressing mechanism and making it possible to seal the members of even relatively thick packing bodies maintaining a high speed and stability (patent document 2).